The present invention generally relates to an electronic detection circuit and more particularly, to a humidity detecting circuit for detecting completion of cooking through the humidity level in a cooker such as a microwave oven or the like.
Such a humidity detecting circuit is incorporated in a humidity detecting device employed in the microwave oven or the like.
The conventional humidity detecting circuit is constructed as shown, for example, in FIG. 1. More specifically, with employment of a first temperature-detecting resistor H' and a second temperature-detecting resistor N', the first temperature-detecting resistor H' exhibits an output voltage VH, through self-heating by a large and constant current source IH. Both of the temperature-detecting resistors H' and N' have linear resistance values with positive temperature coefficients by the use of metallic films., the second temperature-detecting resistor N' exhibits a voltage VN proportional to the ambient temperature by a very small and constant current source IN. When a constant current value is set so that a voltage difference between the two temperature-detecting resistors H' and N' i.e., the value of VN-VH, becomes zero in the dry condition, a difference voltage also becomes zero independent of the ambient temperature. When air contains steam through progress of cooking in the microwave oven, the first temperature-detecting resistor H', self-heated to 150.degree. C. through 200.degree. C., is deprived of heat by the steam, thereby lowering the temperature thereof, and resulting in the voltage across the first temperature-detecting resistor H' being lowered. As the output voltage VN on the side of the second temperature-detecting resistor N' does not change, VN--VH is not zero as a result. This voltage is amplified by a factor of Rf'/RS' by an operational amplifier CP3 to detect the existence of humidity. An operational amplifier CP1 and an operational amplifier CP2 are voltage followers for transmitting the output voltages VN and VH to the operational amplifier CP3.
The following equation is established, where resistance values at 0.degree. C. of the first and the second temperature-detecting resistors H' and N' are respectively RH and RN, temperature coefficients thereof are respectively .alpha.H and .alpha.N, and the resistance values thereof at the temperatures tH and tN are respectively rH and rN.
rH=RH(1+.alpha.H.multidot.tH) (1)
rN=RN(1+.alpha.N.multidot.tN) (2)
Meanwhile, a temperature rise (tH-tN) through self-heating is linearly related to the amount of power consumed by the first temperature-detecting resistor H'.
As tN is equal to the ambient temperature, EQU rH.multidot.IH.sup.2 =hm(tH-tN)S (3).
Hereupon, hm is the heat transfer coefficient and S is a surface area of the first temperature-detecting resistor H'. From the equations of (1) and (3), the following equation can be obtained. ##EQU1## As the antecedent of the equation of (4) becomes a constant when hm is constant in the dry condition, the first temperature-detecting self-heating resistor H' has the following resistance value at 0.degree. C. ##EQU2## Accordingly, the first temperature detecting-resistor H' in this case is equivalent to a temperature-detecting resistor having the temperature coefficient of .alpha.H.
The output voltages of the operational amplifiers CP1 and CP2 in FIG. 1 are respectively as follows. ##EQU3## An output Vout of the operational amplifier CP3 is as follows. ##EQU4## When IN and IH are set so that ##EQU5## may be established to set as a constant, Vout of the equation (7) normally becomes zero as hm is constant under the dry condition. The following equation can be obtained from rewritting the equation (7), ##EQU6## Although hm is constant in the dry condition, hm increases as the steam starts to come out of the cooked food in course of the cooking operation. At this moment, Vout increases rapidly from zero so that humidity detection may be performed. A change of Vout with time is shown in FIG. 2.
In the conventional humidity detecting circuit of FIG. 1, however, it is required to provide a large number of circuit elements such as two constant-current sources IH and IN, three of the operational amplifiers CP1, CP2 and CP3, and the like. Furthermore, it is very difficult to incorporate IH and IN of the two constant-current sources into the circuit so as to meet the conditions the equation of (7).